Television brightness and contrast control circuit



Y i/Ld Jan. 8, 1963 R. w. AHRoNs ETAL 3,072,741

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Jan. 8, 1963 R. w. AHRoNs ETAL 3,072,741

TELEVISION BRIGHTNESS ND CONTRAST CONTROL CIRCUIT Filed March 27, 1959ZSheets-Sheet 2 INENTORJ' RICHARD W. Flr-mums LESLIE- L. BURNSR. 2x

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3,672,741 MELEVSEN AND QNTRAST CNTRL QRCUET Richard Wihred Ahrens,Somerville, and Leslie L. Burns, dr., l); uceton, NJ., assigners toRadio Qorporation of Amen-ca, a corporation of Delaware Fiied Mar. 27,R95?, Ser. No. $92,336 2 Claims. (Cl. 17d-7.5)

This invention relates to an automatic system for controlling thebrightness and contrast of the image reproduced by a television imagereproducing device.

ln order to make the description of the invention that follows clear, itshould be kept in mind that by contrast control is meant the control,generally on a television receiver or monitor, that principally controlsthe amount of peak-to-peak video signal applied to the image reproducingdevice. Similarly, the brightness control operates to vary the amountor1 direct current (DC.) voltage in the signal that is applied to theimage reproducing device. Eecause the low level or dark portions of thepicture are particularly sensitive to the absolute level at which theyoccur, the brightness control is often referred to as a backgroundcontrol.

Brightness and contrast controls are generally included on a televisionreceiver, for example, primarily to enable the viewer to correct forprogram and station differences as well as aging of the receivercomponents. Program differences, created for example by very poor grayscale rendition (resulting when old movies are being viewed), may becorrected to some extent by variation of the brightness and contrastsettings of a television receiver. Since some television transmittingstations operate with more set up, some adjustment of the brightnesscontrol on the usual television receiver is often required to obtain asuitable background of the reproduced image. Also, some stations tend tomodulate at a higher level than others which requires a suitableadjustment of the contrast control.

Although brightness and contrast controls are useful, the average viewerdoes not understand these controls weil enough to be able to obtain anoptimum picture except by a process of trial and error. Both thecontrast and brightness controls interact with each other to the extentthat each alects the operation of the other. This interaction of thecontrols is considered desirable by some engineers but on the other handmay so confuse the viewer that he may not consistently obtain optimumperformance from the television receiver.

it has been found that on the average, if the viewer is not allowed toadjust any of the controls, a more satisfactory picture can be obtained.The reason for this statement may be illustrated by a simple example.Suppose the viewer turns up the contrast to compensate for lowmodulation. Next, suppose he later changes to a station that modulatesfully and observes that the blacks are tco dark and then turns up thebrightness control. Under these conditions, it the scene next changes toonel with high overall brightness, the receiver high voltage powersupply could immediately overload and cause deocussing, blooming or, ifthe television receiver is a color set, turn the entire picture togreen.

Accordingly, it is an obiect of this invention to pro- Fatenteel dan. 8,i963 vide a novel and improved cathode ray tube automatic contrast andbrightness control system.

Current practice in color television receivers and color monitors usingshadow-mask color kinescopes requires operating the liinescope at ornear its maximum ratings for high voltage power input. ln order toprovide good regulation, the high voltage supply should be capable ofproviding considerably more than the safe power input to tne hinescope.A monitor or receiver operating this way can be overloaded and possiblydamaged by the simple condition of applying excessive level of videosignal to the kinescope.

In a typical high voltage supply for a color kinescope, high voltageilyback pulses, derived from the horizontal deflection outputtransformer, are applied to the input electrode of a rectii'ler. Therectitier delivers a charging current to a capacitor (which may be partof the kinescope) connected between the output electrode or" therectifier and a point of reference potential in response to therectification of the ilyback pulses. The DC. voltage developed acrossthe charging capacitor is applied to the kinescope ultor electrode.Since the effective loading on the ultor supply will necessarily varywith picture content, i.e. with beam current, D.C. regulation isgenerally required. Suitable voltage regulation is achieved by shuntingthe space discharge path of a regulator tube across the rectiiier outputcircuit. However, such regulator requires an additional tube that isusually relatively expensive and does nothing toward simplifying thebrightness and contrast controls in the receiver.

Hence another object of the invention is to provide a novel and improvedcircuit for maintaining constant the average cathode ray beam current ina cathode ray tube.

A further object of the invention is to provide a novel and improvedcircuit for automatically controlling the brightness and contrast of thepicture produced by a television reproducing system.

An additional object of the present invention is to provide an improvedsystem that prevents spot blooming in a color television receiver.

Still another object of this invention is to provide a novel system thatprevents ultor power supply overload in a color television receiverwhereby the need for a shunt regulator tube in the high voltage powersupply is eliminated.

An additional object of this invention is to provide a novel automaticbrightness and contrast control circuit for a television signalreproducing device which circuit is insensitive to the aging of thetelevision reproducing device and the control circuit itself.

ln accordance with one form of the invention, two feedback loops areemployed in a television receiver; one to maintain the average currentin the knescope (image reproducing device) constant; the other toprevent the while peaks of the video signal driving the kinescope romexceeding the level at which spot blooming occurs.

As noted above, it is normally desirable to operate a color televisionreproducing device under conditions of maximum contrast and brightness.That is, the contrast control is normally adjusted to the point justbelow where spot blooming becomes objectionable. Also, the brightnesscontrol is normally adjusted to the point where the kinescope ultorpower supply does not overload for the AApril 1954.

' vision signal as the case may be.

. zontal deflection and output circuits 24.

accaniti Y tomatically maintaining these maximums.

A more detailed description follows, in conjunction with theaccompanying drawings, in which like reference numerals refer to likeparts, in which:

FlGURE l is a partial block diagram of a typical color televisionreceiver wherein the average kinescope current is maintained constantand the peak whites of the video signal are held just below bloominglevel;

FGURE 2 is a schematic diagram illustrating the details of the twofeedback loops of FIGURE l; and

FlGUR'E 3 is a partial schematic diagram of an alternative circuit thatmay be used in place of that employed in FIGURE'Z to maintain constantaverage kinescope current.

FIGURE l shows, by way of illustration only, `a typical televisionreceiver which may for example be similar to that described in PracticalColor Television for the Servicerlndustry, published by RCAService'Company lncorporated, Camden, New Jersey, Second Edition,

same as that of the CT C7 chassis, the circuits and service data forwhich are available from RCA' ServicefCom'- pany, Camden 8, New Jersey.While the Vspecific form 'of the signal processing apparatus does notconstitute a Y part of the invention, the showing of a suitable receiveris madeto fully and clearly set forth the environment in which theinvention may operate. TheV ground symbol has been omitted in theseveral blocks for the sake of clarity but may be assumed atV presentwhereneeded to complete a circuit. Y

In FIGURE l, a transmitted color television signal, received by anantenna lll, is applied to the input terminals of a television signalprocessing section 12 of the receiver. This signal processing section 12may include the usual 'radio frequency, mixer, andl intermediatefrequency stages of a typical television receiver. In the alternative,the television receiver signal processing section 12 may be consideredas the input of a composite color television signal from a suitablestudio signal source. In this case, the remainder of the circuitry inFIGURE 1 would then be termed a color television monitor.V The inventionas Y will be described below has equal utilityV with either a receiveror a monitor, monochromeV or color.

The output of the television signal processing section 12 is passed to avideo detector 14 which detects the intermediate frequency signal fromthe processing section 2 to provide a composite black and white or colortele- In the instance of a color television receiver, as hasV beenassumed, the composite color television signal from the video detector14 is passed to the video circuits 16 which may include a luminancechannel and a chrominance channel. ln the chrominance channel, thechrominance signal portion of the composite signal is demodulated andmatrixed to form the several red, green and blue color difference sig- Ynais which are then applied to the respective guns of an imagereproducing device or color kinescope 18.

The video circuit 16 also amplies the luminance and synchronizing pulsesof the composite color television signal. TheV luminance signal, thusprocessed, is then applied to the red, green and blue guns of the colorkinescope 18. The synchronizing components are passed from the videocircuits 16 through a synchronizing signal (sync) separator andautomatic gain control (AGC) circuit 2'@ which operates both toautomatically control the gain of the television signalY processingsection 12 in a well known manner and also drive the vertical deflectioncircuits 22. In addition, the synchronizing components pass through thesync separator 2li and drive the hori- As is well known, the syncseparator 2h also provides a keying pulse tothe chrominance portion ofthe video circuit 1d to enablethe color synchronizing burst to beseparated from In the alternative, the receiver may be the -H thecomposite color television signal in order that it might Lbe used tocontrol the demodulation of the chrorninance signal and thus derive thered, green andblue color difference signals.

The color kinescope 13 includes a deflection yoke 25 having terminals VVfor the vertical and HH for the horizontal windings of the yoke. Thevertical deflection windings VV are coupled to the output terminals VVof the vertical deflection circuits 22.

The horizontal dellection and output circuits 24 drive Va horizontaloutput transformer 2S of the high power voltage supply 36. The terminalsHH of the horizontal deflection windings derive line frequency scanningwaves from output terminals HH of the horizontal output transfa'cross ,asmall segment of this portion.

` potential. 'In this case, the point of xed reference potential is thatof the plate or B-lsupply circuit 36. The lower portion of the highvoltagek transformer 28 includes 'a conventionalV B-boost circuit 33which returns the lower end ofthe horizontal output transformer 2Sthrough a vresistance voltage divider 40 to ground. The series voltagedivider 40 includes a potentiometer 42, the arm of which providesa'variable voltage related to the B-boost voltagcvwhich may be employedas the picture control voltage for adjusting the level of theV constantaverage kinesco-pe beam current as will be described hereinafter.

In this ciriuit the B-boost voltage is a measure of the kine- `scopebeam current owing through the kinescope ultor electrode 44.

The Yhigh voltage for the ultor electrode 44 ofV the kinescope 18 isprovided by high voltage rectier 46 the anode of which is connectedV tothe high potential terminal of the output transformer 28. The ultorsupply electrode 44 is Yconnected to the cathode of the high voltagerectier 46.

'Ihus far described the circuit is that of a conventional televisionreceiver. In accordance with the invention, a

vsmall amount of additional circuitry is added to the conbegins. Thesignal from this detector is ampiiiied, rectitied and then applied tothe automatic gain control circuit of the receiver in such Va manner asto turn down theoverall set gain so as to reduce the amount of spotblooming. Proper choices of the anti-blooming circuit gain, timeconstant, and sensing point are made such that the net eifect is to holdthe brightest portion of a given scene just below spot blooming.

Specifically, the luminance signal from the output of the video circuit1o is passed through a bandwidth limiting circuit 5t) in order toprevent the blooming circuit from setting on narrow noise pulsesextending into the white picture region. The white peaks of theluminance signal, thus bandwidth limited, are then referred to ablooming reference voltage 52 by a blooming detection and amplificationcircuit 5ft. Gnly theV peaks above this reference level are allowed topass and be amplied.

These amplified white peaks are peak detected and the resulting D C.voltage is used to control the video gain of the video circuits 16.Speclc circuitry ior accomplishing this function is described in detailin FlGURE 2. With such system, the output video or luminance signal hasits absolute amplitude controlled so that the White peaks are allowed toexceed the 4blooming reference level by approximately l/ (loop gain) ofthe normal (open loop) amount.

The automatic brightness control loop uses the boosted B voltage fromthe high voltage power supply as an indication of the average kinescopecurrent. When a predetermined kinescope current is thus sensed, beyondwhich the power supply would be overloaded resulting in the rasterbecoming defocussed, the DC. bias of the video output stage is changedin the proper direction to reduce the kinescope current. This controloperates in both directions in such manner that the bias on thekinescopes (brightness) is altered to maintain constant averagekinescope current. Because the kinescope average current remainsconstant, the ultor power supply sees a constant load therebyeliminating the need olr the conventional shunt regulator to be in thepower supply.

Speciticallr, the kinescope current controlling loop receives from thehigh voltage power supply 3h an electrical signal proportional to theamount of average kineseope current flowing. This signal as mentionedpireviously is derived from the arm of the potentiometer ft2 which formsa portion of tlieseries voltage divider l0 connected to the B-boostcircuit 3S. lf this electrical signal, which is proportional to theB-boost voltage, varies from a predetermined setting thereby indicatinga change in the average kinescope current an ultor current controlcircuit 56 operates to vary the bias on the kinescope (which controlsbrightness) 'to maintain constant average kinescope current.

In one form of the invention, the ultor current control circuit Se usesvertical retrace pulses from the vertical deilection circuits 52 whichare allowed to pass through a gate controlled by the B-boost voltage invarying amounts depending upon the value of the r3-boost voltage. Thesevertical retrace pulses are then amplified and detected to obtain thevariable DC. feedback signal which is used to control the bias of thevideo circuits le, which in turn control the bias of kinesccpe i8. Inanother form of this invention, the ultor control circuit Se may usehorizontal retrace pulses.

From this description it may be demonstrate that the redroduced imagewould be somewhat dependent upon the picture content since a picturecontaining, ior example, low peak whites causes the video gain to beincreased due to the operation of the anti-blooming circuit. Because ofthe constant average kinescope current control, the brightness orbackground may be lowered to maintain the average kinescope current at aconstant value. The viewer will have the impression that he is watchingan A.C. coupled receiver. But such receiver will have both freedom fromOverload and also freedom from blooming and at a reduced cost because ofthe omission of the relatively expensive shunt regulator tube.

In FIGURE 2, the details or" a conventional color television receivercircuit (here the RCA CTC 7 referred to above), modied to accommodatethe teachings of the subject invention, is illustrated. in this gure,only the two control loops are illustrated. The constant current loopbegins with the circuitry in the upper left of the diagram. Varyingamounts of the vertical retrace pulses from the vertical deflectioncircuits 22 (FIG. l) are allowed to pass the diode o4- to be amplifiedby triode amplifier 6G and then peak detected by a second diode 65. Thedetected signal from the diode 66 isiiltered by the RC combination edand then used to vary the bias on the control electrode of the secondvideo ampliiier 62. Since the second video amplifier 62 is DC. coupledto the cathode electrodes i0 of the kinescope lo, the bias on the gridof the second video amplier varies the bias on kinescope T3 whichdetermines the average ultor current drawn by the kinescope thuscompleting the constant current loop.

The portion ot the vertical retrace pulses 'that are allowed to pass thediode o4 is determined by 'the value oi the B-boost voltage applied tothe terminal "i2 which is derived from the B-boost circuit 3S (FIG. l).The 'variable potentiometer ft2, which corresponds to that illustratedin PIG. l, provides a picture control whereby the portion of the B-boostvoltage used to bias the diode ed is varied. In this manner, dependingupon the bias applied to the anode of the diode tibi, varying amounts ofthe positive going vertical retrace pulses are allowed to pass to thetriode amplifier dit.

In operation, as the B-boost voltage'drops from a more positive value toa less positive value thereby indicating an increased average kinescopecurrent, the positive bias applied to the anode of the diode tiddecreases thereby allowing a larger portion oi the vertical retracepulses to pass to the triode amplitier oil. These increased amplitudepulses, after amplification, detection, and iil'tering increases thenegative bias applied to the control grid of the econd video amplier o2.This in turn increases the plate voltage of the second video amplifier62 thereby decreasing the average kinescope ultor current to maintainthe average kinescope constant. A similar description of the operationcan be given for the situation where the t3-boost voltage increasesindicating a decrease in average kinescope current.

The anti-blooming circuit receives its input from the plate of thesecond video amplifier 62 in the form of a sync positive signal Vasindicated by the waveform 74. Simply stated, the anti-blooming circuitseparates the white peaks 76, which would cause spot blooming, from therest of the video signal 7dby means of two diodes 725 and Si). Theseparated white peaks 7e are amplified by triode S2 and peak detected bya'diode do to provide a DC. control signal. This control signal iscoupled to the first video amplifier 56 and causes the AGC voltagederived from the plate oi video amplier S6, to decrease the overall setgain, thereby reducing the spot blooming.

The blooming sensing circuit just described has several unique features.To begin with, there must be a blooming reference voltage to which thepeak white signals may be compared in order to ascertain the amount bywhich the video white peaks exceed the blooming level. Spot blooming maybe determined, to a good approximation, with respect to the kinescopered guns grid to cathode voltage. This red gun voltage is in the minus25 to minus 50 volt range in a typical color television set of the CTC7variety. Since the kinescope cathodes in the typical color televisionsets are about Z5() volts above ground, if the red background voltage isaltered by only 10% (about 25 volts) the blooming reference voltage, ifa constant voltage is used, is altered by 50 to 109% as compared to theminus 25 to minus 5G volt grid to cathode blooming` voltage. lt is thusapparent that to have a desirable blooming control circuit operation theblooming reference voltage must track the red background voltage whichmay be changed in order to compensate for the tube aging, etc.

Thus, in accordance with the invention, the blooming reference voltageVR is obtained from the red background potentiometer SS. Thus, changesin the setting of the red background potentiometer SS are reilected inthe blooming voltage VR maintaining a constant blooming reference withrespect to the red grid to cathode voltage. Stated in another manner,the blooming reference level is made to track the red backgroundcontrol. It may be noted in passing that prior to passing to theblooming detector, including the diodes '7S and 859, the video signal'74 is rst passed through a low pass RC filter 9d. This low pass iilterlimits the signal fed to the blooming detector to'about 500 ltilocyclesand effectively reduces the efiect of impulse noise. j

The anti-blooming circuit illustrated in FIG. 2 also includes a novelmethod of altering the direct-current appearing between the grid andcathode of the boot strapped irst video amplifier 86. As will berecalled from-the above description, since the gain of the LF. stages ofa television receiver are already controlled by-an automatic gaincontrol (AGC) system, in order to prevent spot blooming, a very lowfrequency (henceforth called D.C.) control signal is applied to the bootstrapped first video amplier. 'Ehe first video amplifier provides DC.gain to the control Signal which alters the AGC voltage such `as toincrease or decrease the LF. gain of the television receiver in theappropriate direction. However,l inserting a DC. control signal into aboot strapped video arnplifier is an extremely dirhcult problem. Y

More specically, the problem is to insert the DC. control voltage, whichis referred to grid-to-cathode of first video amplifier, without: (l)having the boot strapped amplifier cathode resistor attenuate the D C.gain ofthe stage as Vin a cathode follower, (2) having the video outputsignal peak detected in the DC. control circuitl thereby causingerroneous DC. control, (3) A.C. shorting the boot-strapped cathoderesistor, (4) A.C. grounding the LF. thus reducing the video gain tounity, (5)

the DC. from the boot strapped video biasing the diode in the controlcircuit.

circuit illustrated in FIG. 2 overcomes these difficulties. Each ofthese difculties are now considered in succession by describing themanner in which they are overcome by the illustrated circuit. Thecathode resistor 94 or the boot strapped rst video amplifier 86V doesnot effectively reduce its DI. gain because the majority or the D.C.control from the diode 8d appears across the resistor R2 which iscoupled between the cathode Vand control electrode of the videoamplifier 86. The second diiiiculty referred to above is alleviatedsince the video output signal cannot appear across the diode 84. VTheresistors R3 and R4 and capacitors C3 and Criorm a balanced network sothat bothy terminals of the diode-84 are balanced with the sameproportion of the video output signal. ln this connection the capacitorC2 bypasses the resistor R2 for video frequencies to allow equalproportions of the video output signal to be applied equally to bothterminals of diode S4. The third difficulty mentioned' above isalleviated since the resistor R4 is much greater than the cathoderesistor 94. This means that the cathode resistor 94 is not A.-C.shorted to ground by the capacitor C4. YThe fourth difficulty mentionedabove is alleviated since the resistor R2 is much greater than thecathode resistor 94 and also since the capacitor C2 bypasses theresistor R2 for video frequencies thus preventing the video from the LF.amplifiers from being A.C.

grounded by the capacitor C3. The fifth difficulty mentioned above isalleviated since Vthe capacitors C3 and C.; prevent any DC. biasing ofthe diode 84 by the boot strapped video stage.

in FiG. 3 there is illustrated an alternative arrangement which may beused in order to maintain constant average kinescope currentin lieu ofthat circuit portion of FIG. 2 which is enclosed in the dottedrectangle. In accordance with the circuit of FIG. 3 a sample of thekinescope current is obtained and the voltage this current producesacross resistor 99, is subtracted from a reference voltage it?. Afiltered voltage proportional to the difference is then applied to thecontrol grid of the second video amplier d2 (FIG. 2) and fed to thecathode electrodes 7i? (FIG. 2) of the guns of the color kinescope i8.in this manner, the average kinescope current is maintained at aconstant value.V

ln FG. 3, lthe horizontal output tube from the block 2d (PEG. 1) drivesa high voltage and deliection transformer 96 whichhas an isolated highvoltage winding 9S. ln this manner, the kinescope ultor current iscaused to flow through a potentiometer 99 to a voltage reference pointist?. The potentiometer 99.is variable and constitutes a picture controlpotentiometer. The voltage drop occurring across the potentiometer 99-subtracts from the voltage appearing at the voltage reference pointwhich, for example, may be B+, and the difference is applied to the gridof the secondtvideo amplilier 62 (FlG. 2)

As in the case of the circuit operation of FIG. 2 this diierence outputsignal is applied to the cathodei) (FIG. 2) of the color kinescope iS(FiG. 2) thereby varying the kinescope current. This complete circuitforms a highly degenerative feedback loop which Will reduce thedeviation of the average kinescope current, due to a changing videosignal, by l/loop gain; Because'the gain of the second video amplifier,theV loop gain is suficicntly high so that the average kinescope currentmay be considered a constant.

Ey way of example, to understand the circuit operation let it be assumedthat the average kinescope current increases. This results in increasedcurrent ilow through the potentiometer 99. The voltage from the grid tocathode of the second video amplifier 62'(FIG. 2) drops. With reducedvoltage at its controlV grid, the plate of the second video amplifier 62(FG. 2) increases in a positive direction thereby decreasing thekinescope current. By this technique the assumed increase in averagekinescope current is nulliiied by the subsequent increase resulting fromthe feedback, thereby maintaining an essentially co-nstant'averagekinescope current. Y

A novel'and improved televisionsystemV hasbeenvdescribed which includesnot only a feature for maintaining the average kinescope constant butalso includes antibloorning circuitry for preventing spot blooming fromoccurring due to peak Whites in the video signal. This invention resultsin a relatively low cost circuit which eliminates the need forregulating the power supply voltage and also provides a circuit which isquite easy for the inexperienced viewer to operate under optimum viewingconditions. The circuits of this invention find utility in eithermonochromecr color television receivers and monitors.

What is claimed is:

`l. In a television system including a kinescope having a beam intensitycontrolling electrode and an ultor electrode, a high voltage supplyincluding a source of yback pulses and a rectifier for rectifying saidflyback pulses having an output electrode coupled to said ultorelectrode to provide an electron beam, said high voltage supply alsoincluding a circuit for obtaining a B-boost voltage, said B-boostvoltage being inversely proportional to the average current passing tosaid ultor electrode from said high voltage'supply, mans to deflect saidbeam in horiizontal and vertical directions, and means for applying avideo signal to said beam intensity controlling electrode to intensitymodulate said beam in accordance with said video signal, an automaticbrightness control system comprising means including said B-boostcircuit for sensing the current passing to said ultor electrode fromsaid high voltage supply, said deflecting means including a source ofvertical retrace pulses, and means coupled between said sensing meansand said source Vof Vertical retrace pulses for passing varyingamplitude portions of said retrace pulses as a function of said B-boostvoltage, means coupled to said variable passing means for rectifyingsaid varying amplitude portions of said retrace pulses to obtain a DC.voltage proportional to said average ultor current, and means coupled tosaid D.C. voltage deriving means for controlling the direct currentle'vel of said beam intensity controlling electrode whereby to maintainthe average ultor current constant.

2. ln a color television signal receivingsystern, the combinationcomprising, a color ltinescope, signal translating means for processinga received signal to provide a video signal, a video signal amplifierfor coupling said signal translating means to said kinescope, meanscoupled with said kinescope for sensing the average kinescope beamcurrent owing therein, including a high voltage supply circuit for saidkinescope providing a B-boost voltage Signal that is proportional to theaverage kinescope beam current, means responsive to at least a portionof said B-boost voltage signal for altering the bias on the guns of saidkinescope, including a source of vertical retrace pulses, amplitudeselective means coupled between said source of retrace pulses and saidhigh voltage supply to pass portions of said retrace pulse-s that varyin an amount proportional to said B-boost voltage, and means coupled tosaid amplitude selective means for peak detecting the passed portions ofsaid retrace pulses to provide a D.C. control voltage inverselyproportional to kinescope beam current.

References Cited in the le of this patent UNITED STATES PATENTS KellJuly 3, 1951 Schwarz Mar. 16, 1954 Preisig June 5, 1956 Rhodes Apr. 29,1958 Schade July 15, 1958 Sericht Nov. 25, 1958 FOREIGN PATENTS BelgiumOct. l, 1956 Germany Ian. 14, 1960 OTHER REFERENCES RCA Color TelevisionReceiver, chassis No. CTCS; Service Data 1956, No. T4, rst printing5-29-56.

2. IN A COLOR TELEVISION SIGNAL RECEIVING SYSTEM, THE COMBINATIONCOMPRISING, A COLOR KINESCOPE, SIGNAL TRANSLATING MEANS FOR PROCESSING ARECEIVED SIGNAL TO PROVIDE A VIDEO SIGNAL, A VIDEO SIGNAL AMPLIFIER FORCOUPLING SAID SIGNAL TRANSLATING MEANS TO SAID KINESCOPE, MEANS COUPLEDWITH SAID KINESCOPE FOR SENSING THE AVERAGE KINESCOPE BEAM CURRENTFLOWING THEREIN, INCLUDING A HIGH VOLTAGE SUPPLY CIRCUIT FOR SAIDKINESCOPE PROVIDING A B-BOOST VOLTAGE SIGNAL THAT IS PROPORTIONAL TO THEAVERAGE KINESCOPE BEAM CURRENT, MEANS RESPONSIVE TO AT LEAST A PORTIONOF SAID B-BOOST VOLTAGE SIGNAL FOR ALTERING THE BIAS ON THE GUNS OF SAIDKINESCOPE, INCLUDING A SOURCE OF VERTICAL RETRACE PULSES, AMPLITUDESELECTIVE MEANS COUPLED BETWEEN SAID SOURCE OF RETRACE PULSES AND SAIDHIGH VOLTAGE SUPPLY TO PASS PORTIONS OF SAID RETACE PULSES THAT VARY INAN AMOUNT PROPORTIONAL TO SAID B-BOOST VOLTAGE, AND MEANS COUPLED TOSAID AMPLITUDE SELECTIVE MEANS FOR PEAK DETECTING THE PASSED PORTIONS OFSAID RETRACE PULSES TO PROVIDE A D.C. CONTROL VOLTAGE INVERSELYPROPORTIONAL TO KINESCOP BEAM CURRENT.